


Arthur Pendragon's Formal Investigation into the Strange Matter of Aithusa and Merlin

by injeong



Series: Merlin at Hogwarts [1]
Category: Harry Potter - J. K. Rowling, Merlin (TV)
Genre: Aithusa is a cat, Alternate Universe - Harry Potter Setting, Alternate Universe - Hogwarts, Arthur is suffering, Cat Aithusa, Crack, Fluff, Harry Potter AU, Humor, Implied immortal Merlin, M/M, Merthur - Freeform, implied reincarnation?, or is she
Language: English
Status: Completed
Published: 2021-02-08
Updated: 2021-02-08
Packaged: 2021-03-14 11:39:43
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,998
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29295288
Author URL: https://archiveofourown.org/users/injeong/pseuds/injeong
Summary: Aithusa was Merlin's pet. A white, fluffy, lithe cat that had an uncanny habit of appearing wherever Merlin was. Arthur was also convinced that she was a dragon in disguise.AKA A Hogwarts AU in which Aithusa is maybe a dragon is disguise and Arthur is getting a headache.
Relationships: Merlin/Arthur Pendragon (Merlin)
Series: Merlin at Hogwarts [1]
Series URL: https://archiveofourown.org/series/2178123
Comments: 7
Kudos: 196





	Arthur Pendragon's Formal Investigation into the Strange Matter of Aithusa and Merlin

There was something wrong with Merlin's cat. 

Arthur hadn't trusted it since the moment he first laid eyes on it, staring at him with its big, glowing blue eyes through the glass of the compartment window on the Hogwarts Express. He could have _sworn_ that he heard a voice saying "My King", but that was ridiculous, because there was nobody else in the carriage corridor and all the doors to the various compartments were closed. 

He ended up sitting in the same compartment as a tall, dark-haired boy, who was apparently the cat's owner. Arthur shifted uncomfortably in his seat every time the cat - Aithusa, the boy said - slowly turned to look at him, blinking serenely. In an attempt to distract himself, Arthur had ended up in a conversation with the boy, who introduced himself as Merlin, and got himself into a lengthy and heated debate about the qualities of the different Hogwarts Houses and how the Headmaster was so fantastically biased towards Gryffindor. ("What's so good about encouraging young kids to be stupidly reckless?" Merlin had argued, and Arthur responded with "It's not _recklessness_ , Merlin, it's being brave and chivalrous, not that you would know!")

Aithusa had then interrupted with a small miaow, and Merlin barely glanced at her, saying, "Oh, thanks, Aithusa," and rummaging through his bags. Arthur watched in confusion as he pulled out a mass of black fabric - the Hogwarts robes - and started tugging them on over his Muggle clothes. 

"What are you doing?" 

Merlin raised an eyebrow. "We're five minutes away from arriving. You should get changed too." 

"But how did you know -" Arthur glanced out of the window. The smoke from the train almost obscured the blurred view of the countryside, and it was impossible to see whether the castle was in sight or not. He looked suspiciously at Aithusa, who blinked innocently back at him. 

"Did the cat -"

But no, that was ridiculous.

Merlin quirked an eyebrow, and Arthur cleared his throat awkwardly, looking away. 

It was impossible for a cat to look smug, but Arthur was pretty sure the expression on Aithusa's face was exactly that. 

  
Arthur had almost hoped to never see the two of them again, but as luck would have it, he was sorted into Gryffindor, Merlin was sorted into Hufflepuff and they ended up attending the same lessons almost every day. 

"I swear you're not allowed to bring your pet into lessons," Arthur whispered over his cauldron, and Merlin glared daggers at him, motioning violently for him to shut up and almost knocking over his vial of armadillo bile. 

Weirdly, the Potions Professor didn't seem to notice the Very Obvious white cat slinking around the dungeon, licking at random bits of chopped liver dropped onto the floor and occasionally chasing after a mouse. Was the cat invisible? Was the Professor just turning a blind eye? But why would he? The Professor was ridiculously strict and definitely would have taken at least fifty points from Hufflepuff if one of his students were blatantly breaking the school rules and bringing a cat of all things into a Potions class. 

Arthur's head was starting to hurt. 

  
"So," Arthur said to Merlin. "School trip to see dragons, huh?" 

"Dragons," said Merlin, his eyes shining with excitement. On his lap, Aithusa squirmed and said something like _mrrawoow_. Arthur eyed it cautiously. 

"Did the Headmaster say we could bring pets?" 

"Shh!" 

And then, of course, Merlin went on to accidentally release Aithusa into a dragon compound full of two-hundred-pound winged fire-breathing beasts. Before Arthur could have a heart attack, he realised that Aithusa was not immediately snatched up and eaten, but was instead purring and rubbing up against a particularly massive golden dragon, who bent down and nudged it with its nose. 

_Greetings again, young one_ , it said, and _smiled_. 

Arthur's jaw dropped. 

Aithusa went _mroourw_ , and the dragon settled down and fixed Arthur with an amused look. 

_Hello again, young king._

_What the fuck what the fuck whatthefuck -_

Merlin grabbed Arthur's elbow as his legs buckled. 

"Arthur? What's wrong? Are you hurt?" 

Scrambling away from the dragon compound, Arthur said shakily, "T-the dragon -"

"What?" 

"It _talked_." 

There was a long, weighty pause, where Merlin's face went several shades of white and pink and red and eventually gave an awkward, shrill sort of laugh. 

"No it didn't." 

"It _did_ ," Arthur insisted desperately. "It - it called Aithusa young one, and it said hello to me and then called me young king - for goodness' sake, Merlin, look at it, it's _grinning_ , how the fuck is it doing that -" 

Merlin promptly turned on his heel and manhandled Arthur back to the Professors, loudly saying "Arthur, I think you should go to a Healer, hearing things is never a good sign -" 

Needless to say, Arthur spent the next week in the hospital wing and blamed Merlin for every hour of it. Merlin visited and brought Aithusa and Arthur almost screamed at the sight of the white cat with blue eyes. 

_Mraow_ , she said, and sat up on the end of his bed and started licking her paw, the perfect picture of feline innocence. 

Arthur didn't trust it one bit. 

The next thing that tipped Arthur off was the discovery that Merlin, the boy who went everywhere (and he meant _everywhere_ ) with his cat, was in fact ... allergic to cats. 

"You're joking," Arthur said flatly. Merlin let out a pitiful sneeze. 

"What do you mean?" he said thickly. "I'm not allergic. I'm just ... this is just hay fever." 

"Merlin, it's winter," Arthur said. 

Merlin fumbled awkwardly and tried to pat Mithian, the long-haired Balinese cat that lived in one of the Pendragon estates. Arthur had dragged Merlin to his house over the holidays (because Merlin had stayed at school every single holiday and it was just getting depressing) and Ygraine had welcomed them both with open arms. Mithian was Ygraine's cat, a gift from Uther, and Merlin hadn't even stepped through the front door before he began sneezing. 

On the floor, Mithian pressed up to Merlin and head-butted his leg affectionately. Merlin gave her a watery smile, and sniffed loudly. 

"I'm not allergic," he said firmly, his voice already starting to sound blocked up, and Arthur resisted the urge to sigh. 

"How are you allergic to cats? Aithusa is literally around you all the time! She goes with you to the shower, for god's sake -" 

And really, it should have all been very suspicious, and maybe Arthur should have asked more questions and tried harder to get the truth out of his friend, but at that moment Merlin started wheezing and Arthur had to bodily drag him from the room. 

  
"There's something about you," said Arthur suspiciously. Aithusa sat on his bed and gave him an unimpressed look. 

"Don't look at me like that. You're not a cat, are you? Merlin's deathly allergic, you know that? If you really were a cat, he'd be dead by now." Although he had read something in a Muggle newspaper about cats for allergic people ... but there was no such thing as a truly hypoallergenic cat, was there? 

Aithusa stretched lazily over his blanket. 

"I'm going to find out, you know." Arthur pointed at the cat and tried not to feel embarrassed at himself. "Don't think I won't. I'm going to be watching you." 

Blinking sleepily, Aithusa started licking her fur. 

_Miaowr_ , she said sarcastically, and Arthur clenched his fists. 

It had been a long day, and an even longer night - why did Arthur sign up to be a Prefect? He _knew_ they had to patrol the school corridors at night - and Arthur was ready to creep back into his dorm and fall onto the covers and go to sleep, when he heard voices from the boy's bathroom several feet away. Curious, he stepped closer - it was two in the morning, what sort of weirdo decided to go all the way downstairs to the bathroom when every House dormitory had their own toilets?

"- not sure how much longer I'll be able to keep this up," a voice said miserably. Arthur barely refrained from tripping over his feet. It was Merlin. 

What the hell was he doing there? A Hufflepuff, sneaking out of his room at some ungodly hour in the morning?

Arthur was just about ready to storm in and ask him what the hell he was doing when another voice spoke up. 

"Patience," it said, and Arthur almost tripped again. It was a female voice, and it sounded strange - both light and heavy, deep and soft and high all at the same time, but it was a female voice - what was a girl doing in the men's bathroom? Rather, what was Merlin doing with a _girl_ in the men's bathroom at two in the bloody morning? 

Intrigued, Arthur edged closer. 

"It's been ages," Merlin said desperately. "And - I know I can't tell him, and even if I could I probably wouldn't be able to figure out how - but he's just there, all the time, and at the same time he's not - what am I supposed to do? He's not the same person he was before, obviously, but he's still exactly the same and it's like I know him but I don't - it's so confusing, Aithusa, I don't know what to do." 

Aithusa?? 

"The time will come, my lord," the girl said consolingly. "The Once and Future King has risen, and with time, he shall return fully. Of course, the fact's that he's here means that Albion is in its greatest need, which is concerning, but I'm not too worried about that right now." 

Merlin sighed. "Well, at least you're not as cryptic as Kilgharrah. God, if I was stuck with him for another thousand years, I would have gone mad." 

_Thousand years_? Arthur thought blankly. And who the hell was Kilgharrah?

"I would not have anyone else to be stuck with for these thousand years," said the girl fondly, and Merlin let out a small laugh. "Kilgharrah probably just likes being cryptic because he has a reputation to uphold." 

"Yeah, a reputation as a bloody giant lizard liar," said Merlin sullenly, but with a hint of amusement. "Thanks, Aithusa. You're right. I just got ... overwhelmed, I guess. It's two in the morning, we should go back." 

There was a student called Aithusa? Arthur frowned, trying to remember the voice. He hadn't heard it before. He ducked into a nearby corridor as Merlin shuffled out of the toilets, looked around, and hurried back towards the Hufflepuff common rooms.

That had been very weird. 

Arthur waited for the owner of the mystery voice to come out, but she never did. Befuddled, Arthur returned to the Gryffindor rooms and went to sleep.

Several hours later, he bolted upright in his bed. 

There had been no girl. Aithusa was the owner of the voice. 

  
"You - you can _talk_ ," he said in a strangled voice, pointing at the white cat. They were in the Hufflepuff dormitory, and Merlin had gone downstairs to grab some more rolls of parchment for their Transfigurations essay. 

Aithusa mreeow-ed at him innocently. 

"You can understand me, can't you? Oh my god, you can _talk_ -" 

There was a rustling of curtains, and Arthur jumped back as another Hufflepuff boy peered out at him in confusion. Will, one of Merlin's friends, Arthur thought. 

"What are you doing, mate?" 

Arthur flushed. "Nothing." 

Grinning, Will leaned closer. "Really? 'Cause it looks to me like you're trying to talk to that cat." 

"No, I wasn't," insisted Arthur. He glanced out of the corner of his eye at the cat, and scowled at it. 

Then there was the incident at Hogsmede. After the death of Lord Voldemort, the Death Eaters had scattered, most of them rounded up and neutralised, but there were still some supporters left in hiding, biding their time, trying to gather up enough forces to stage an attack. It was just Arthur's luck that he and Merlin happened to be in town that one day. 

"We have some sort of crazy bad luck," Arthur hissed, dragging Merlin behind a shop as the hooded figures flooded the streets. Flashes of light darted through the air, rebounding off the walls. 

"You tell me," said Merlin miserably, using his wand to extinguish his sleeve, which had caught fire after it had been hit by a stray spell. He was remarkably calm about this, Arthur noted hysterically - _how_?? 

There really wasn't much they could do. They hadn't even begun learning how to Apparate, yet, and Muggle technology didn't work in Hogsmede so they couldn't call for help. Arthur tightened his grip on his wand. 

"Do you know any secret ways out of Hogsmede, perhaps?" he asked Merlin desperately. Merlin, surprisingly, brightened. 

"I do, actually! There's one in -" Arthur watched in alarm as Merlin cut off, his eyes widening in panic. "Arthur, _watch out!_ -" 

The next few seconds passed in a blur of terror and shock and confusion. Arthur was shoved to the ground by Merlin as a hooded figure skidded round the corner and rapidly fired off several bolts of emerald light, and Merlin crumpled on the spot as the curses hit their mark. Arthur had been almost blind with panic, stumbling towards his fallen friend and barely noticing the Death Eater being driven away by a sudden, roaring burst of flame that seemingly came out of nowhere, and only when Merlin opened his eyes and said, "Fucking _ow_ , what was that?" did he sit back, the fear seeping away, and let loose a breath he didn't know he had been holding. 

" _Don't_ do that again," he half-yelled, and Merlin winced. 

"Jeez, okay. I'm fine, see? There's a secret exit in Honeydukes that leads back to Hogwarts. Come on, let's go." 

Arthur only noticed when they were halfway through the dark, damp underground corridor that Aithusa was running alongside them. 

"When did she get here?" 

Merlin looked down, and frowned. 

"Um. I think she found us right when that Death Eater hit me?" 

Arthur looked at the white cat, and the way her eyes glowed almost luminous in the dark, and the faint smell of smoke that hung around her, and slowly started to edge away. 

"On a completely unrelated note," he whispered to Merlin once they were safely back in the Great Hall, "Can your cat breathe fire?" 

"What?"

"Never mind." 

  
Arthur, at one point, had spent a solid four days in the school library, searching through every book on magical creatures he could find. (There were a lot. He was starting to feel sorry for the people who took Care of Magical Creatures.) And yet, he still couldn't find anything on telepathic white cats who could breathe fire and talk to dragons. 

"You've been here for a while," the librarian, Professor Monmouth, said suspiciously. "Anything in particular you need?" 

Arthur looked guiltily at the seven-foot high pile of books on his desk. 

"Um. Books on ... fire-breathing telepathic shape-shifting magical creatures?" 

Professor Monmouth looked at him blankly, and Arthur went red. 

"Sorry, sir. Maybe something on, um, dragons?" 

Still giving him suspicious looks, Professor Monmouth led him over to the correct section and left him staring at the shelves full of dusty books that looked like they weighed more than the dragons they were about. 

"Your attempts are valiant," the white dragon said to him, sitting leisurely in the middle of the Gryffindor Common room. Arthur froze awkwardly, still half-inside the portrait hole. 

He blinked, then blinked again. The dragon didn't disappear. 

"Um," he said eloquently. "Who - who are you?" 

The dragon sniffed. "I think you know who, young king." 

"I - um, don't?" Arthur stared, and suddenly realised that the Common room was empty. There was no sounds from the dormitories upstairs - the dormitories who were meant to be full of rowdy, loud-mouthed Gryffindors who usually engaged in heated, yelled debates late into the night. He paled. "Wait. Did you eat all my friends?" 

To his astonishment, the dragon started laughing. 

"Ew," it said. "Humans are so ... stringy. I've never liked the taste of them." 

" _You've tasted humans before_??" 

"No, I was being sacastic. I'm a dragon, I'm not an _animal_." 

Arthur stared at it blankly. "Uh -" 

The dragon raised an eyebrow - since when did dragons have eyebrows? - and gave him an unimpressed look. Its eyes were very blue, Arthur noticed. Blue like Merlin's. Blue like -

"No way." 

The dragon, whose scales were pearly white like Aithusa's fur, grinned smugly. 

"No _fucking_ way -" 

"I have no idea what you're talking about," Merlin squeaked nervously. On his lap, Aithusa went _mrraoow_. 

"Don't test me," said Arthur dangerously, pointing at the cat, then turned back to Merlin. "Surely you must know. You almost died because you were so allergic to Mithian when you came over during the holidays. I know I'm right!" 

Looking hilariously panicked, Merlin gulped, his eyes darting wildly everywhere and determinedly looking at anywhere but Arthur. 

"Um. I have a - a very reasonable explanation for all of this." 

Donning his best you're-not-being-very-convincing expression, Arthur sat back and crossed his arms. 

"I'm waiting." 

Merlin laughed nervously. "Well, um, you see ... the white dragon you thought you saw in the Gryffindor common room ... was a hallucination." 

Well. At least he tried. 

Arthur raised an eyebrow. "A hallucination, you say?" 

"Yes!" Merlin nodded vigorously, and Aithusa grumbled in discontent. "I made a hallucinogenic potion and slipped it in your drink." 

"You ... made a potion ... to make me see dragons." 

"Well. Now that you say it out loud ..." 

Arthur buried his face in his hands. 

"A dragon?" Morgana scoffed. "Arthur, I know you want to live a life of adventure, but imagining your boyfriend's cat into a fire-breathing dragon is going a bit far." 

"I'm telling the truth," wailed Arthur. Morgana was raising an eyebrow at him as she worked on her History of Magic essay. 

"You don't deny that you're dating, then? Good. Gwaine owes me twenty Galleons." 

Arthur spluttered, blushing furiously, and Morgana grinned her evil grin. 

**Author's Note:**

> I wrote this a while ago and completely forgot to post it whoops  
> Hogwarts Merthur!! I love sticking the characters from Merlin into a Harry Potter setting, it's always so fun to do xD There's an angstier Hogwarts AU fic in my drafts, so once I edit that a bit I'll post it too ^^


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